code to read emscripten_metadata section of Web Assembly files

emscripten-metadata.js

function lebReader(array) {
  const view = new Uint8Array(array);
  let at = 0;

  return () => {
    if (at >= view.length) {
      return -1;
    }

    let ret = 0;
    let mul = 1;
    for (;;) {
      const byte = view[at++];
      ret += ((byte & 0x7f) * mul);
      mul *= 0x80;
      if (!(byte & 0x80)) {
        return ret;
      }
    }
  };
}

export function metadataRead(module) {
  const dylink = WebAssembly.Module.customSections(module, 'dylink');
  const dylinkReader = lebReader(dylink[0]);
  const memorySize = dylinkReader();
  const memoryAlign = Math.pow(2, dylinkReader());
  const tableSize = dylinkReader();
  const tableAlign = Math.pow(2, dylinkReader());

  const metadata = WebAssembly.Module.customSections(module, 'emscripten_metadata');
  const metadataReader = lebReader(metadata[0]);

  const metadataMajor = metadataReader();
  const metadataMinor = metadataReader();
  if (metadataMajor || metadataMinor) {
    throw new Error(`unexpected metadata version: ${metadataMajor},${metadataMinor}`);
  }

  metadataReader();  // ignore ABI major
  metadataReader();  // ignore ABI minor

  const memory = metadataReader();
  const table = metadataReader();

  return {memory, table};
}